Optical interference is a phenomena used widely throughout the sciences. In particular, the use of short-coherence (or ‘low-coherence’) interferometric imaging has become an important imaging modality in several fields and notably in medical applications. In interferometric imaging, light from a known and controlled optical path (the ‘reference path’) is caused to interfere with light returned from an unknown path such that information about this unknown path (the ‘sample path’) may be determined by an analysis of the resulting interferogram.
In short-coherence imaging, the interferogram contains the depth location information of structures within the sample being analyzed. Scanning short-coherence light over a sample volume to produce tomographic images is known as Optical Coherence Tomography, or OCT. In recent years, practical laser-based light sources with coherence lengths of 20 μm or less have become available, promoting the use of OCT in several fields including ophthalmology, general microscopy, cardiology and oncology.
A particular advantage of OCT is its inherent compatibility with fiber optics making it a nearly ideal imaging modality for non-invasive or minimally invasive medical procedures.
Central to all OCT implementations is the requirement that the lengths of the sample and reference paths be matched to ensure the interference effect being recorded corresponds to a desired scan region within the sample. In the case of relatively long optical catheters required in many procedures (approximately 1.5 to 2 meters is common) this requirement for matching the path lengths may become difficult to achieve, especially when many practical implementations of OCT require matching on the millimeter scale. Furthermore, the very thin fibers used in these catheters can easily stretch or contract several millimeters during use.
When using OCT in any application, the optical ‘zero-point’ is critical. This defines where, in the image space, the so-called reference plane exists. By convention, surface planes are in the x-y plane, and the depth occurs along the z-axis. In a microscope application for example, it may be beneficial to set the zero point at the surface of the microscope slide, so specimens can be measured against this known surface. In a catheter inserted into a bodily lumen, the most useful reference plane is the outer surface of the catheter tip itself, and all distances are measured outward from this location.
For a rotating catheter, the x-y-z space is best represented in polar coordinates (angle and radial distance). Hence the z-axis becomes the radial distance from the center. Practically, setting a match point means that the optical length from the chosen reference plane in the sample is equal to the primary optical length in the reference arm. The high speed changing of the reference arm length in scanning represents only a small variation on the primary length. Because OCT penetrates tissue only a few millimeters at most, the scan is practically limited to typically 1-5 mm, whereas the actual lengths of the sample and reference arms may be several meters.
For example, in the case of optical catheters used in cardiology, the instrument itself will be located outside the nominal ‘sterile field’ surrounding the patient, the catheter itself will be inside this field, and an umbilical will be used to join the two. The total optical length of the sample arm (catheter plus umbilical) can easily approach 5 meters, which will also be the primary length of the reference arm. Since scanning will be perhaps 5 mm, this represents 0.1% of the total length. Measurement accuracy is required to be 0.1 mm or better in this application. Since it is not cost-effective to control the lengths of each optical fiber within the catheter and umbilical to sub-millimeter dimensions, most design approaches use an adjustable reference path within the optical imaging equipment to adjust to each catheter as it is used.
However, a medical application may use many disposable catheters per day; all interfaced to the same imaging equipment. Thus, while the primary path length adjustment can work quite effectively, it usually requires an initial adjustment by a skilled operator who understands the optical reflection pattern or ‘signature’ of the catheters that will be recorded by OCT to determine how to adjust the reference path to coincide with the outer surface of the catheter tip. Again, the adjustment of the image zero-point, or reference plane location is performed by adjusting the primary path-length of the reference arm. This adjustment is often termed ‘z-offset’ of the reference arm and is controlled via a motor, called simply the z-offset motor. By convention, the instrument z-offset is zero when the sample arm length (catheter) is manufactured exactly as designed; is negative when the catheter is too short; and positive when the catheter is too long.
These optical catheters typically have a lens and reflector structure placed at their distal tip to focus and direct the light for scanning purposes. The light typically propagates through one or more transparent sheaths that comprise the catheter outer structure. Each of the interfaces can and will cause a reflection that will be detected by OCT. Hence, it may be challenging to determine which of those reflections corresponds to the desired optical reference point (‘zero-point’) of the system. Since measurements are made based on this zero-point setting, setting it correctly can have significant importance in medical applications. Furthermore, because there may be several closely spaced and similar intensity reflections, the use of software to detect the proper zero-offset (‘z-offset’) is extremely problematic and unreliable.
As noted, the optical fiber can stretch significantly on these scales when the catheter is advanced or retracted. For example, using the known yield strength of standard optical fibers used in OCT, and the catheter length, it is easy to show that 10 mm of stretch can occur before the fiber breaks. Typical forces encountered in real situations will only cause a 1 mm stretch or less, but many medical measurements require accuracy of ¼ millimeter or better.
Therefore, a simple, cost effective method for reliably determining the optical match point (‘zero-point’) of the catheter is needed. Furthermore, this method should be compatible with real-time video rate imaging so that the zero-point can be tracked as the catheter is maneuvered, retracted or advanced. The present invention addresses these issues.